Understanding CH-Stretching Raman Optical Activity in Ala-Ala Dipeptides

Raman optical activity (ROA) becomes a standard method to monitor peptide conformation. However, the signal in the CH-stretching region is particularly difficult to measure and interpret. In order to understand the structural information contained in this part of the spectrum, data obtained on a custom-made ROA spectrometer have been analyzed for the model Ala-Ala molecule, with the help of molecular dynamics (MD) and density functional theory computations. The Ala-Ala enantiomers provided the "mirror image" spectra, which proves that the signal can be reliably measured, in spite of a rather low ROA/Raman intensity ratio (∼2 × 10^-5). The theoretical modeling indicated that the most intense ROA bands can be attributed to locally asymmetric CH₃ and ^αCH vibrations, whereas symmetric methyl CH-stretching modes contribute less. A simplified model made it possible to estimate the contribution of local chirality of the two alanine residues to the resultant ROA pattern. In spite of a significant frequency shift (over 100 cm^-1) because of the anharmonic corrections, the harmonic level was able to explain the main spectral features. The anharmonic corrections were treated by second-order perturbation and limited vibrational configuration interaction procedures. This allowed for assignment of some weaker spectral features because of the combination and overtone vibrations. The results show that the peptide CH-stretching ROA signal contains rich structural information, reflecting also the peptide environment. The experimental data, however, need to be deciphered by relatively complex and time-consuming spectral simulations.